I grew up on MA/NH border and would x/c ski and mt bike (before fat bikes) on the snowmobile trails. During the day it is quite safe. Even at night I had no problems. As a precaution I would wear something bright (hunters vest). The thing about following established routes are that many restaurants along the routes have easy access, which means a place to stop to warm up and eat.
Links bellow are for NH. There are many local snowmobile clubs that maintain trails. I would look into them and see about their use policies. I did most of my riding in north central MA and south western NH. Also some of these routes use logging roads and abandoned rail road beds which also make good riding in warmer months.
http://www.nhstateparks.org/uploads/pdf … or_Map.pdf
http://www.nhstateparks.org/experience/ … fault.aspx